1) Field of the Invention
The invention relates to a method of bumped wafer thinning, and more particularly, to a method of wafer thinning by a mechanical back grinding method.
2) Description of the Prior Art
Conventional bumped wafer thinning starts with a bumped wafer that is laminated to a back grind (BG) tape to the front side of the wafer followed by backside wafer thinning. FIG. 1 illustrates the bumped wafer thinning process. The silicon wafer 100 has solder bumps 150 formed on a top side of the wafer and laminated to BG tape 120. The top of the wafer 100 is held to a vacuum chuck 130. Grinding wheel 140 is shown. The grinding wheel will thin the wafer from the backside. Challenges in conventional bumped wafer thinning include the following. 1.) Successful ultra thinning of a bumped wafer largely depends on the availability of back grind (BG) tape respective to the bump geometry. BG tapes are available in the market for bump heights in the range of 100–200 μm. For bump heights greater than 200 μm it is difficult to manufacture BG tapes with thicker cushioning layer thickness and hence thinning becomes very difficult. 2.) Total Thickness Variation (TTV) control largely depends on the tape selection, tape lamination process without any air traps, and optimized BG and polishing processes. 3.) FIG. 2 summarizes the issues with conventional bumped wafer thinning.
1) Stressed solder balls 210 and UBM 212. During the mechanical back grinding process the solder balls and UBM are stressed if proper tape selection is not done.
2) Mechanical Force difference. The mechanical grinding process exerts a certain amount of force onto the silicon wafer in order to physically machine the silicon surface. With the bumped wafers, there is always a difference in force exerted above the bump area 214 and the region without bumps 216.
4) Surface Irregularities 218. The difference in the force exerted on the region with and without bumps leads to formation of dimples (surface irregularities) on the back side of the wafer. With ultra thinning of bumped wafers the surface irregularities become the potential area of failures.
5) Contamination due to adhesives 220. In the BG tape lamination process, the tape adhesive with cushioning layers are squeezed between the bumps. During de-taping, remains of adhesives may be left on the bump surface. These tape contaminants adversely affect the Flip Chip interconnection process.
The paper “Key Challenges in Fine Pitch Bumped Wafer Mechanical Back Grinding and Polishing”, by Ganesh V P, V Kripesh, Kumaran Pakiri, and George Itoh, 2003 International Conference on Electronics Packaging, pp 260–265, Tokyo, Japan, discusses these issues.
A number of patents address thinning bumped wafers. Most of the reported methods use resin/wax coated to the active side of the wafer with the bumps and taped before thinning the wafer. U.S. Pat. No. 6,338,980 to Satoh describes a method of thinning bumped wafers for WLP and chip scale package (CSP) applications. The advantages of this method include thinning without edge damage and normal handling. However, there are too many process steps, including ashing and resin curing, which require process optimization for different bump heights. The dicing operation must be performed twice. Furthermore, proprietary resins are used. U.S. Pat. No. 6,500,764 to Pritchett shows a method for thinning bumped wafers. This process results in good TTV control, but requires additional process steps for forming dummy bumps. Since the wafer has dummy bumps, tape lamination cannot seal the edges resulting in grinded silicon particle and slurry contamination issues. U.S. Pat. No. 6,638,837 to Tsao et al discloses a method for applying an organic passivation layer or an aqueous material for protection of the active surface, front side, of semiconductor wafers during the operations of backside grinding, transporting, and packaging. Disadvantages of this process include additional process steps and higher materials cost. The process method for depositing the liquid material will have a lot of waste which will have impact on the cost.